Ultrasound signs of renal and ureteral pathology

Non-visualizing kidney

If any kidney is not visualized, repeat the examination. Adjust sensitivity for clear visualization of the liver and spleen parenchyma and scan in different projections. Determine the size of the visualized kidney. Renal hypertrophy occurs (at any age) several months after the removal of the other kidney or its cessation of functioning. If there is only one large kidney, and the second is not detected even with the most careful search, then it is possible that the patient has only one kidney.

If one kidney is not detected, keep the following in mind:

1. The kidney may have been removed. Check the patient's medical history and look for scarring on the patient's skin.
2. The kidney may be dystopic. Examine the kidney area, as well as the entire abdomen, including the pelvis. If the kidney is not found, then do an X-ray of the chest organs. Intravenous urography may also be needed.
3. If one large but normal kidney is detected, if there was no previous surgical intervention, then congenital agenesis of the other kidney is quite likely. If one kidney is visualized, but it is not enlarged, then the lack of visualization of the second kidney suggests the presence of a chronic disease.
4. If there is one large but displaced kidney, this may be a developmental anomaly.
5. Failure to visualize both kidneys may be due to changes in the echogenicity of the kidneys as a result of chronic renal parenchymal disease.
6. A kidney less than 2 cm thick and less than 4 cm long is poorly visualized. Locate the renal vessels and ureter, this may be helpful in locating the kidney, especially if the ureter is dilated.

A pelvic kidney may be mistaken for a tubo-ovarian mass or gastrointestinal tumor on sonography. Use intravenous urography to confirm the position of the kidney.

Big kidney

Bilateral enlargement

1. If both kidneys are enlarged but have a normal shape, normal, increased or decreased echogenicity, and a homogeneous echostructure, the following possible causes should be considered:
   • Acute or subacute glomerulonephritis or severe pyelonephritis.
   • Amyloidosis (more often with increased echogenicity).
   • Nephrotic syndrome.
2. If the kidneys have a smooth outline and are diffusely enlarged, have a heterogeneous structure, and increased echogenicity, then the following possible causes must be taken into account:
   • Lymphoma. May produce multiple areas of low echogenicity, especially Burkitt's lymphoma in children and adolescents.
   • Metastases.
   • Polycystic kidney disease.

Unilateral increase

If a kidney is enlarged but has normal echogenicity, and the other kidney is small or absent, the enlargement may be the result of compensatory hypertrophy. If one kidney is not visualized, it is necessary to exclude crossed dystopia and other developmental anomalies.

The kidneys may be slightly enlarged as a result of congenital lobulation (doubling) with two or three ureters. Examine the renal hilum: two or more vascular pedicles and ureters should be visible. Intravenous urography may be necessary.

One kidney is enlarged or has a more lobular structure than normal

The most common cause of kidney enlargement is hydronephrosis, which is presented on echograms as multiple rounded cystic zones (calyces) with a wide centrally located cystic structure (the width of the renal pelvis normally does not exceed 1 cm). Sections in the frontal plane will demonstrate the connection between the calyces and the pelvis. In multicystic kidneys, such a connection is not detected.

Always compare the two kidneys when measuring the renal pelvis. When most of the renal pelvis is outside the renal parenchyma, this may be a normal variant. If the renal pelvis is enlarged, the normal echostructure is impaired due to tight filling of the pelvis with fluid.

Enlargement of the renal pelvis may occur with hyperhydration with increased diuresis or with bladder overflow. The renal calyces will be normal. Ask the patient to urinate and repeat the examination.

Dilation of the renal pelvis may occur during normal pregnancy and does not necessarily mean inflammatory changes. Check urine for infection and the uterus for pregnancy.

Enlarged renal pelvis

An enlarged renal pelvis is an indication for examination of the ureters and bladder, as well as the other kidney, to identify the causes of obstruction. If the cause of dilation is not identified, excretory urography is necessary. Normal, concave-shaped calyces may acquire a convex or rounded shape as the degree of obstruction increases. Accordingly, the renal parenchyma becomes thinner.

To determine the degree of hydronephrosis, measure the size of the renal pelvis with an empty bladder. If the pelvis is more than 1 cm thick, then no expansion of the calyces is determined, there are initial signs of hydronephrosis. If there is dilation of the calyces, then there is moderate hydronephrosis; if there is a decrease in the thickness of the parenchyma, then hydronephrosis is pronounced.

Hydronephrosis can be caused by congenital stenosis of the ureteropelvic junction, stenosis of the ureter, for example, in schistosomiasis, or in the presence of stones, or by external compression of the ureter by retroperitoneal formations, or formations in the abdominal cavity.

Kidney cysts

If ultrasound reveals multiple, anechoic, well-demarcated areas throughout the kidney, polycystic kidney disease may be suspected. Multicystic kidney disease is usually unilateral, whereas congenital polycystic kidney disease is almost always bilateral (although cysts may be asymmetrical).

1. Simple cysts can be single or multiple. In ultrasound examination, cysts have a rounded shape and smooth contour without internal echostructure, but with a distinct enhancement of the posterior wall. Such cysts are usually single-chambered, and in the presence of multiple cysts, the sizes of the cysts vary. Rarely, these cysts become infected or hemorrhage occurs in their cavity, and internal echostructure appears. In this case, or if there is an unevenness of the cyst contour, additional examination is required.
2. Parasitic cysts usually contain sediment and are often multilocular or septate. When the cyst calcifies, the wall appears as a bright echogenic convex line with acoustic shadowing. Parasitic cysts may be multiple and bilateral. Also scan the liver for other cysts, and perform a chest X-ray.
3. If multiple cysts are detected in the kidney, it is usually enlarged. Alveolar echinococcus may be detected. If the patient is under 50 years old and there are no clinical manifestations, examine the second kidney to detect polycystic disease: congenital cysts are anechoic and do not have parietal calcification. Both kidneys are always enlarged.

More than 70% of all kidney cysts are a manifestation of benign cystic disease. These cysts are common in people over 50 years of age and can be bilateral. They rarely cause clinical symptoms.

Kidney tumors

Ultrasound cannot reliably differentiate benign renal tumors (other than renal cysts) from malignant renal tumors and does not always accurately differentiate malignant tumors from renal abscesses.

There are two exceptions to this rule:

1. In the early stages, renal angiomyolipoma has pathognomonic sonographic features that allow an accurate diagnosis. These tumors can occur at any age and can be bilateral. Sonographically, angiomyolipoma is a well-defined, hyperechoic, and homogeneous structure, and as the tumor grows, dorsal attenuation appears. However, tumors with central necrosis have marked dorsal enhancement. At this stage, differential diagnosis by ultrasound is not possible, but abdominal radiography can reveal fat within the tumor, which is virtually uncommon in any other tumor type.
2. If a kidney tumor invades the inferior vena cava or paranephric tissues, then it is undoubtedly malignant.

Solid tumors of the kidneys

Renal tumors may be well-demarcated or may have unclear borders and deform the kidney. Echogenicity may be increased or decreased. In the early stages, most tumors are homogeneous; in the presence of central necrosis, they become heterogeneous.

It is important to be able to differentiate between normal or hypertrophied columns of Bertin and a renal tumor. The echotexture of the cortex will be the same as that of the rest of the kidney; however, in some patients the differentiation may be difficult.

Formations of mixed echogenicity with heterogeneous echostructure

Differential diagnosis in the presence of heterogeneous formations can be very difficult, but if there is extension of the tumor beyond the kidney, there is no doubt that it is malignant. Malignant tumors may not extend beyond the kidney. Both tumors and hematomas can produce acoustic shadowing as a result of calcification.

As the tumor grows, its center becomes necrotic, and a structure of mixed echogenicity with an uneven outline and a large amount of internal suspension appears. Differentiating a tumor at this stage from an abscess or hematoma can be difficult. In order to establish the correct diagnosis in this case, it is necessary to compare the ultrasound picture and clinical data. Tumors can spread to the renal vein or inferior vena cava and cause thrombosis.

Always scan both kidneys if you suspect renal cancer (at any age), scan the liver and inferior vena cava. Also do a chest x-ray to rule out metastases.

An echogenic mass with an uneven, undermined outline, containing a suspension against the background of an enlarged kidney, may be a malignant tumor or a pyogenic or tuberculous abscess. Clinical data will help differentiate these conditions.

In children, malignant tumors such as nephroblastoma (Wilms' tumor) are well encapsulated but may be heterogeneous. Some have calcifications, but not along the capsule. Hemorrhages or necrotic changes may alter echogenicity. Some tumors are bilateral.

Little kidney

1. A small kidney with normal echogenicity may result from renal artery stenosis or occlusion or congenital hypoplasia.
2. A small kidney of normal shape, a hyperechoic kidney may indicate chronic renal failure. In chronic failure, both kidneys are likely to be affected.
3. A small hyperechoic kidney with an uneven, scalloped outline, with uneven parenchyma thickness (usually bilateral changes, but always asymmetrical), often occurs as a result of chronic pyelonephritis or an infectious lesion such as tuberculosis. Abscesses may contain calcifications, which are defined as hyperechoic structures.
4. A small, normal-shaped, hyperechoic kidney may occur in the late stages of renal vein thrombosis. Acute renal vein thrombosis usually causes enlargement of the kidney with subsequent scarring. Chronic obstructive nephropathy may also produce similar changes in one kidney, but the changes in chronic glomerulonephritis are usually bilateral.

Kidney stones (calculi)

Not all stones are visible on plain radiography of the urinary tract, but not all stones are visible on ultrasound either. If clinical symptoms suggest the presence of a stone, all patients with a negative ultrasound examination should undergo intravenous urography.

Suspected presence of urinary stones, abnormal urine tests, but negative ultrasound results - intravenous urography.

Stones are most clearly visible in the collecting system of the kidneys. The minimum size of a stone that can be visualized using general ultrasound equipment with a 3.5 MHz transducer is 3-4 mm in diameter. Smaller stones (2-3 mm) can be detected using a 5 MHz transducer. Stones appear as hyperechoic structures with an acoustic shadow. Stones should be visualized in two different projections, longitudinal and transverse, to determine their exact location and take measurements. This will help to avoid false-positive diagnoses in the presence of calcifications in the renal parenchyma and other tissues, such as the neck of the calyces, which can simulate stones by creating a similar hyperechoic structure with a shadow.

Ureteral stones are always very difficult to detect using ultrasound. The inability to visualize a ureteral stone does not mean that it is not present.

Injury

1. In the acute stage, echography may reveal intrarenal or pararenal anechoic areas due to the presence of blood (hematoma) or urine extravasation.
2. When blood clots are organized and thrombi are formed, hyperechoic or mixed echogenicity structures with anchogenic inclusions appear (mixed echogenicity formation or formations). In all cases of injury, examine the opposite kidney, but remember that ultrasound cannot determine renal function.

The ability to visualize a kidney does not mean that the kidney is functioning. Use intravenous urography, radionuclide studies, or laboratory tests to determine renal function. Remember that renal injury can cause temporary loss of function.

Perinephric fluid collection

Blood, pus and urine near the kidney cannot be differentiated by echography. All of this appears as anechoic zones.

Retroperitoneal formations

Lymphomas are usually para-aortic and aortocaval lesions. If the sensitivity level is low enough, they may appear fluid-filled. Any such lesion may displace the kidney.

A psoas abscess or hematoma may be anechoic or have mixed echogenicity: blood clots are hyperechoic. In the presence of gas, some areas may be hyperechoic and produce an acoustic shadow.

Adrenal formations

Scan both adrenal glands. Adrenal masses may be primary or metastatic tumors, abscesses, or hematomas. Most have a clear border, but some are poorly differentiated. Hematomas are most common in neonates.

The inability to visualize the adrenal gland does not exclude the presence of pathology in it.

Ureters

Because of the deep position of the ureters behind the bowel, it is very difficult to visualize normal ureters using ultrasound. In the presence of dilation (for example, in case of obstruction due to prostatic enlargement or urethral stricture or due to vesicoureteral reflux), the ureters are better visualized, especially near the kidney or bladder. The middle third of the ureter is always difficult to visualize, although intravenous urography is much more informative. However, in the presence of wall thickening, for example in schistosomiasis (in some cases with calcification), the ureters are easily visualized using echography.

The lower third of the ureters can be visualized when scanning through a filled bladder, which creates a sufficient acoustic window.

Ultrasound is not a reliable method for detecting either ureteral stones or stenosis.

Differential diagnosis of kidney diseases

Single large cyst

• Rule out giant hydronephrosis.

Irregularity of the kidney contour (except for lobulation)

• Be aware of the possibility of chronic pyelonephritis or multiple renal infarctions.

Unevenness of the kidney contour (smoothed)

• Normal lobulation or cystic disease (congenital or parasitic).

Non-visualizing kidney

• Extopia or displacement.
• Surgical intervention.
• Too small in size for echographic visualization.
• Tumor displacement.

Large kidney (normal shape)

• Hydronephrosis.
• Cystic disease.
• Acute renal venous thrombosis.
• Compensatory hypertrophy (the other kidney is absent or shrunken).

Large kidney (asymmetrical shape)

• Tumor.
• Abscess.
• Parasitic cyst.
• Polycystic disease in adults.

Little kidney

• Glomerulonephritis.
• Chronic pyelonephritis.
• Infarction or chronic renal venous thrombosis.
• Congenital hypoplasia.

Perinephric fluid *

• Blood.
• Pus.
• Urine.

*Ultrasound cannot differentiate between these types of fluid.

Non-visualized kidney? Always check the contralateral kidney and look for the kidney in the pelvis.